1. Field of the Invention
The present invention relates to a method, circuit and program for driving plasma display panels.
2. Description of the Related Art
Plasma display panels have many features of: (1) a thin structure and a less flickering, (2) a high contrast ratio, (3) relatively large-area screen, (4) a fast response speed, and (5) a self-emissive type and a capability of multi-color emission by using fluorescent material. Therefore plasma display panels are widely used in the computer related fields of display devices and color image displays.
At the moment there are strong demands for plasma display panels to increase brightness (a higher brightness) and to increase display contrast (a higher contrast).
Plasma display panels are classified into two types depending on the operation modes: an AC type where electrodes are coated with dielectrics and operated indirectly in an AC discharge manner; and a DC type where electrodes are exposed in the discharge space and operated in a DC discharge manner. The AC type plasma display panels are classified into: a memory operation type where the memory operation in the display cells is used as the driving mode; and a refresh operation type where the memory operation is not used. The brightness of the plasma display panel is in proportion to the frequency of discharges, that is the number of repetitions of application of pulse voltage. The refresh type AC plasma display panel, of which brightness drops as the display capacity increases, is primarily used for a small display capacity plasma display panel.
FIG. 25 is a perspective view depicting a general configuration of the AC type plasma display panel.
The AC type plasma display panel is comprised of a front substrate which faces the user (viewer) side, and a back substrate which positions at the far side of the user.
The front substrate further comprises an insulating substrate 101 which is made of glass, first transparent electrodes 103a which are disposed with spacing on the insulating substrate 101 in the horizontal direction of the panel, second transparent electrodes 104a which are disposed on the insulating substrate 101 so as to face the first transparent electrodes 103a, trace electrodes (bus electrodes) 105 which are disposed overlaying the first transparent electrodes 103a extending in the horizontal direction (lateral direction) of the panel, trace electrodes (bus electrodes) 106 which are disposed overlaying the second transparent electrodes 104a extending parallel to the trace electrodes (bus electrodes) 105, a dielectric film 110 which is formed on the insulating substrate 101 so as to cover the first transparent electrodes 103a, the second transparent electrodes 104a and both of the trace electrodes 105 and 106, and a protective layer 112 made from magnesium oxide which is formed on the dielectric film 110 to protect the dielectric film 110 from discharge.
The trace electrodes 105 and 106 are electrodes with about a 1-10 μm thickness, comprised of CrCu thin film and Cr thin film, and are disposed for decreasing the electric resistance value between the first transparent electrodes 103a and the second transparent electrodes 104a and an external drive device.
The electrodes comprised of the first transparent electrodes 103a and the trace electrodes 105 are called scanning electrodes 103, and the electrodes comprised of the second transparent electrodes 104a and the trace electrodes 106 are called common electrodes (sustaining electrodes) 104.
The back substrate is comprised of an insulating substrate 102 made of glass, a plurality of data electrodes 107 which extend in a direction perpendicular to the scanning electrodes 103 and the common electrodes 104 on the insulating substrate 102, a dielectric film 113 which is formed to cover the data electrodes 107 on the insulating substrate 102, a plurality of barriers 109 which are formed on the dielectric film 113 with spacing for partitioning the display cells, and a fluorescent material 111 formed on the exposed face of the dielectric film 113 and on the side wall of each barrier 109.
A discharge gas space 108 separated by barriers 109 is formed between the front substrate and the back substrate. In this discharge gas space 108, discharge gas containing helium, neon, xenon or a mixed gas thereof is filled. The fluorescent material 111 converts ultraviolet generated by the discharge of this discharge gas into visible light. This visible light reaches the user via the transparent insulating substrate 101.
Now the writing select type drive operation of a conventional plasma display panel constructed as in the above description will be described.
The plasma display panel operates according to the sub-field method. The sub-field method is a method of dividing one field constituting a screen into a plurality of sub-fields (SF) and driving the plasma display panel for each sub-field.
FIG. 26 is a diagram depicting the relationship between one field and sub-fields.
As FIG. 26 shows, 1 field is divided into 8 sub-fields (SF1-SF8), and each sub-field is comprised of 5 periods: a priming (hereafter “priming” may be abbreviated to “Pr”) period; a priming (Pr) erase period, a writing period, a sustaining period and sustaining erase period.
Hereafter it is assumed that the reference potential of the scanning electrodes 103 and the common electrodes 104 is the sustaining voltage Vs, and a potential higher than the sustaining voltage Vs is positive polarity and a potential lower than the sustaining voltage Vs is negative polarity. It is also assumed that the reference potential of the data electrode 107 is the ground potential GND, and a potential higher than the ground potential GND is positive polarity, and a potential lower than the ground potential GND is negative polarity.
FIG. 27 is a timing chart depicting the writing select type drive operation of the plasma display panel shown in FIG. 25. FIG. 28 to FIG. 37 are diagrams depicting the wall charges forming status after each of the abovementioned 5 periods complete.
In the priming period at the beginning of each sub-field, saw tooth wave Pr pulses Ppr-s are applied to the scanning electrodes 103, and rectangular wave Pr pulses Ppr-c are applied to the common electrodes 104. The potential difference between the saw tooth wave Pr pulse Ppr-s and the rectangular wave Pr pulse Ppr-c is set such that the potential difference is larger than that of the discharge start voltages or more of the surface discharge and the counter discharge. Therefore the surface discharge between the scanning electrodes 103 and the common electrodes 104, and the counter discharge between the scanning electrodes 103 and the data electrodes 107 are generated.
The Pr pulses Ppr-s to be applied to the scanning electrodes 103 are saw tooth waves, so the generation and the stop of discharge are repeated according to the rise of the Pr pulses Ppr-s, as stated in Technical Report of IEICE (THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS) EID 98-95, (1999-01), pp. 91-96. Therefore the emission intensity is weaker than the subsequent discharge, that is the writing discharge and the sustaining discharge.
However the priming discharge (also called a pre-discharge or reset discharge) is generated in all the display cells whether an image is displayed or not, so emission by this priming discharge corresponds to the background brightness, that is black brightness. As the voltage gradient of the saw tooth wave Pr pulses Ppr-s becomes smaller, the black brightness decreases, but if the voltage gradient becomes too small, the time required to reach the voltage necessary for the priming discharge becomes long, and as a result the priming period becomes long. Then it is unavoidable to decrease the sustaining period, and as a result the sustaining discharge count decreases and the brightness of the white display drops, which drops contrast. Therefore to balance these elements, a voltage gradient of about 4V/μ seconds is normally used.
By this priming discharge, active particles (priming particles) to generate the discharge of display cells more easily are generated, and at the same time, as shown in FIG. 28, wall charges with negative polarity are attached on the scanning electrodes 103 and wall charges with positive polarity are attached on the common electrodes 104.
In the priming erase period after the priming period, saw tooth wave Pr erase pulses Ppe-s with negative polarity are applied to the scanning electrodes 103. By applying these pulses, a discharge with a weak emission intensity is generated, just like the priming discharge, and as a result the surface discharge between the scanning electrodes 103 and the common electrodes 104, and the counter discharge between the scanning electrodes 103 and the data electrodes 107 are generated. Because of this, the negative polarity wall charges near the scanning electrodes 103, the positive polarity wall charges near the common electrodes 104, and the positive polarity wall charges near the data electrodes 107, generated in the Pr period, decrease as shown in FIG. 29.
The increase and decrease of the wall charges are relatively shown by the number of wall charges shown in each figure. For example, in FIG. 28 the number of negative polarity wall charges near the scanning electrode 103 is 24, the number of positive polarity wall charges near the common electrode 104 is 15, and the number of positive polarity wall charges near the data electrode 107 is 9, but in FIG. 29, these have been decreased to 18, 12 and 6 respectively.
By generating wall charges in this way, the writing discharge can be generated more easily in the subsequent writing period. If wall charges are not adjusted in the priming erase period, a surface discharge is generated between the scanning electrodes 103 and the common electrodes 104 even if data pulses Pd are not applied in the writing period, since very many wall charges have been generated in the priming period, so the possibility of an erred display increases.
The writing period after the priming erase period is a period for selecting the display cells to be emitted, and during this writing period, the potential of the scanning electrode 103 is held at the scanning base potential Vbw, the positive polarity rectangular wave pulses Pw-c are applied to the common electrodes 104, and the potential of the common electrodes 104 is held at the first bias voltage Vsw1, except during the period when the scanning pulses Pw-s are applied. The negative polarity scanning pulses Pw-s with potential Vw are linearly and sequentially applied to the scanning electrodes 103 for each line to be scanned.
On the other hand, positive polarity data pulses Pw-d are applied to the data electrodes 107 synchronizing with the scanning pulses Pw-s according to the display cells to be selected.
When the scanning pulses Pw-s and the data pulses Pw-d synchronize, a writing discharge is generated only in the display cells at the intersection of the scanning electrode 103 and the data electrode 107 to which these pulses are applied, and the wall charges shown in FIG. 30 are attached.
Whereas a writing discharge is not generated in display cells to which the data pulses Pw-d are not applied, so wall charges after priming erase discharge (see FIG. 29) are held in these display cells.
The sustaining period is a period of time for light emission for display, and negative polarity sustaining pulses Psus-s and Psus-c, which start with the common electrodes 104 side and are alternately applied to the scanning electrodes 103 side and the common electrodes 104 side, are applied to the scanning electrodes 103 and the common electrodes 104. In this sustaining period, the sustaining pulse which is applied first is called the first sustaining pulse, the next sustaining pulse is called the second sustaining pulse, and the sustaining pulse which is applied last is called the final sustaining pulse.
In the display cells in which a writing discharge was generated during the writing period, positive charges are attached to the scanning electrode 103, and negative charges are attached to the common electrode 104, and the negative polarity sustaining pulse voltage Vs to the common electrode 104 and a wall charge voltage are superimposed, voltage after superimposing exceeds the surface discharge start voltage, and a surface discharge is generated.
If a surface discharge is generated, wall charges are located so as to cancel voltage which is being applied to the scanning electrode 103 and the common electrode 104 respectively as shown in FIG. 31. In other words, negative charges are attached to the common electrode 104, and positive charges are attached to the scanning electrode 103. Since the next sustaining pulse is a positive voltage pulse at the scanning electrode side, an effective voltage to be applied to the discharge gas space 108 exceeds the discharge start voltage by superimposing with the wall charges, a discharge is generated, and wall charges are generated as shown in FIG. 32. In the wall charges generated by the surface discharge between the scanning electrodes 103 and the common electrodes 104, polarity is switched between the scanning electrodes 103 and the common electrodes 104 each time the sustaining pulse is applied.
The amount of the wall charges in the display cells in which writing was not performed during the writing period, on the other hand, is so small that a sustaining discharge is not generated even if sustaining pulses are applied. Therefore the wall charges after the priming erase period completes, shown in FIG. 29, are maintained as is.
When the surface discharge start voltage and the counter discharge start voltage of a plasma display panel are compared, the counter discharge start voltage is generally higher than the surface discharge start voltage. Because of this, when the first sustaining pulses are applied, the surface discharge is generated but the counter discharge is not. Therefore after the first sustaining pulses are applied, the status of the wall charges near the data electrode 107 is the same as the status after the writing discharge completes.
However by repeating the sustaining discharge, the wall charges for the amount of voltage exceeding the surface discharge start voltage are stored on the scanning electrode 103 and the common electrode 104, so the wall charges increase more than those in writing. Because of this, the negative polarity wall charges of the data electrode 107, the positive polarity wall charges of the scanning electrode 103 and the common electrode 104, and the sustaining pulse voltage Vs exceed the counter discharge start voltage, and a counter discharge is also generated, and as a result the positive charges are stored in the data electrode 107 as shown in FIG. 33. And if the sustaining discharge continues to be repeated, the wall charges to be formed near the scanning electrode 103 and the common electrode 104 also saturate (become a steady state), so positive polarity wall charges to be formed near the data electrode 107 remain unchanged, and the wall charges shown in FIG. 34 and FIG. 35 are formed.
In the final sustaining erase period, the saw tooth sustaining erase pulses Pse-s with negative polarity are applied to the scanning electrode 103, and the rectangular wave pulses Pse-c with positive polarity are applied to the common electrode 104. As the sustaining erase pulses Pse-s decrease, a weak surface discharge is generated between the scanning electrode 103 and the common electrode 104, and a weak counter discharge is generated between the scanning electrode 103 and the data electrode 107 respectively. By this, a part of the wall charges of the display cell, which emitted during the sustaining period before the sustaining erase period, are erased as shown in FIG. 36 and FIG. 37.
In order to drop the black brightness in the abovementioned driving method for the plasma display panel, a method for creating sub-fields in which the priming period and the priming erase period are not set and a method for dropping the emission intensity of the priming discharge, that is a method for decreasing the potential difference between the saw tooth wave Pr pulses Ppr-s and the rectangular wave Pr pulses Ppr-c, are possible.
FIG. 38 shows an example of the timing chart based on the former method.
As FIG. 38 shows, according to this method, the sub-field SF(N+1), in which the priming period and the priming erase period are not set, is created after the sub-field SF(N), in which the priming period and the priming erase period are set.
Here after the sub-field in which the priming period and the priming erase period are not set may be called “Pr skipped SF”, and the sub-field in which the priming period and the priming erase period are set may be called “Pr included SF”.
An example of this method for setting Pr skipped SF and driving the plasma display panel is a method stated in Japanese Patent Application Laid-Open No. 2001-255847. According to the method stated in this document, a sub-field in which not only the priming period and the priming erase period but also the sustaining erase period is not set are created.
However the locations of the wall charges just before the writing period of Pr skipped SF are locations of wall charges after the sustaining erase discharge completes (FIG. 36) if the previous sub-field is emitted, and are locations of wall charges after the Pr erase discharge completes (FIG. 29) if the previous sub-field is not emitted, and especially when display is executed in the previous sub-field, the amount of positive charge stored near the data electrode 107 is low. Since the writing discharge is executed in this status, the minimum emission voltage Vd_min (minimum voltage among voltages with which all the display cells are emitted when the data voltage Vd is increased, normally a voltage higher than this minimum emission voltage Vd-min is set) of the display cell which emitted in the previous sub-field becomes higher than the minimum emission voltage Vd_min in the non-display cell. Or the minimum voltage Vsw1_min of the first bias voltage Vsw1 of the common electrode 104 increases since the writing discharge when the display cell is emitted in the previous sub-field becomes weak.
In other words, the minimum voltage values of the set values of the data voltage Vd and the first bias voltage Vsw1 of the common electrode 104 increase, so the drive margin drops.
The wall charges formed near the data electrode 107 depend on the number of sustaining pulses in the sub-field, and especially when the number of sustaining pulses is low, the negative polarity wall charges formed in writing are more likely to remain.
In the case of the method for dropping the emission intensity of the priming discharge, on the other hand, the wall charges to be stored near the scanning electrode 103, the common electrode 104 and the data electrode 107 decrease to be less than the wall charges shown in FIG. 28, as the potential difference between the saw tooth wave Pr pulse Ppr-s and the rectangular wave Pr pulse Ppr-c decreases, so the minimum emission voltage Vd_min and the minimum voltage Vsw1_min of the first bias voltage Vsw1 of the common electrode 104 increases, as mentioned above.
Also in the sustaining erase period in the sub-field SF(N) in FIG. 38, the potential difference between the scanning electrode 103 and the common electrode 104 decreases if the potential of the common electrode 104 is set to the sustaining voltage Vs, so the wall charges that remain near the scanning electrode 103 and the common electrode 104 are more than the wall charges in the case when the common electrode 104 is set to the first bias voltage Vsw1, and a discharge error easily occurs due to the potential difference between the scanning electrode 103 and the common electrode 104 in the writing period. If the sustaining voltage Vs is increased in this status, the voltage Vs_max with which a discharge error occurs decreases to be lower than the potential Vsw1 of the common electrode 104, and the drive margin drops in this drive waveform as well.
If the drive margin drops, it becomes difficult to absorb the characteristics difference due to the process dispersion of the plasma display panel. Therefore the wider the drive margin the better, but the decrease of the black brightness and the increase of the drive margin are in a trade-off relationship, as described above, so in a conventional plasma display, it is difficult to implement both a decrease of the black brightness and an increase of the drive margin.